Set @file-tree-error-color in the light theme variables (#16954)

GitOrigin-RevId: 128b84af3292a8ac48e5b1010787ec965de835a8

services/web/frontend/stylesheets/variables/themes/light.less

@@ -33,6 +33,7 @@
 @file-tree-multiselect-bg: @blue;
 @file-tree-multiselect-hover-bg: @blue-dark;
 @file-tree-droppable-bg-color: @neutral-40;
+@file-tree-error-color: @ol-blue-gray-5;
 
 @content-alt-bg-color: @neutral-10;